Effects of Temperature and Solvent

Activity on the Viscoelastic Response of

Nafion® for PEM Fuel Cells

Christine Ranney

An Undergraduate Thesis

Presented to Professor Jay Benziger

Princeton Chemical Engineering Department

April, 2007

i

To Barclay

ii

Abstract

This thesis will investigate the mechanical properties of Nafion®, specifically with

respect to viscoelastic creep and membrane swelling, under a range of temperatures and

solvent activities. Studies have shown that the mechanical properties of the membrane are

extremely important to the performance and longevity of the fuel cell. This paper will focus

on a relatively new Polymer Electrolyte Membrane fuel cell (PEM) fuel cell technology, the

Direct Methanol Fuel Cell (DMFC). Nafion®, a perfluorinated ionomer, is the most commonly

used membrane in these fuel cells.

Nafion® is a viscoelastic material, which means that it responds to stress in a time-

dependant manner. The membrane of a fuel cell is subject to various stresses, including

those due to solvent mass uptake and clamping between flow plates, which causes Nafion

to creep. Creep can result in membrane thinning and pinhole formation, which increase the

incidence of methanol crossover, the leading failure mechanism in DMFC’s. Temperature

and solvent activity strongly affect the viscoelastic response of Nafion. Thus, an accurate

knowledge of the mechanical properties of the membrane under a range of environmental

conditions is crucial to improved design and performance of the fuel cell.

The viscoelastic response of Nafion was measured over a range of temperatures and

methanol activities in a specially designed apparatus using viscoelastic creep. Additionally,

the effect of drying time and temperature on creep results were examined. Finally,

membrane swelling dynamics and equilibrium membrane swelling were investigated, as

membrane swelling is closely related to creep in fuel cell membranes.

B It was found that, in general, increased temperature increases creep by decreasing the

bonding strength between polymer strains. However, the combined effects of temperature

iii

and solvent activity are more complicated. It was shown that at room temperature

increased methanol activity causes greater creep. This increase is due to the increased free

volume which occurs with the addition of methanol, and thus increased mobility of side

chains, which causes more creep to occur. At intermediate temperatures 50 and 60 o

C, a

local maximum in creep was seen at a very low methanol activity. These results are similar

to those found by Majsztrik for Nafion creep in water, in which a local maximum at

intermediate activities was found for these temperatures. In methanol, the position of the

local maximum with activity has simply shifted. These results were explained by solute

induced changes of the microphase separation in Nafion. As temperature and solute activity

change, the hydrophilic microphase restructures itself. Less creep occurs in phases in which

the hydrophilic and hydrophobic phases are more separated.

Nafion’s mechanical properties were also shown to be strongly dependent on

thermal history. Drying temperature affects creep by causing rearrangements in the

microstructure of Nafion which may or may not return to its original state upon cooling. The

results of the drying process are a mix of thermodynamics and kinetics, and are thus also

dependent on drying time.

The final morphology of Nafion is a function of thermal history, temperature, solvent

type, and solvent activity. It was found that all of these factors have a significant effect on

the viscoelastic response of Nafion®. Results were explained based on changes in

microstructure and interactions within Nafion®.

iv

Acknowledgements

I would like to begin by thanking my advisor, Professor Jay Benziger, for all of his help and

support throughout the writing of this thesis. Professor Benziger gave me a lot of freedom

to work under my own direction while also offering generous help when it was needed. He

was always incredibly quick to respond to emails and provide input on my work. I also

gratefully acknowledge the financial support and use of lab space I received from Professor

Benziger.

I would also like to acknowledge all the members of the Benziger lab group. It was a

pleasure to get to know and work with Erin, May Jean, Josh, Makini, and Ellazar, as well as

to work alongside fellow seniors Roxy, Hannah, and Taylor. Thanks for letting me into lab

when I was locked out, helping me with Labview, and putting up with me turning off the

lights when I left the room when they were in there working.

Perhaps the most important acknowledgement is to Paul Majsztrik, without whom this

thesis would not be possible. Paul’s dissertation was an invaluable source for this thesis,

providing both motivation for my topic and explanation for some of my results. Paul also

provided the initial instruction and direction for my experimental work. He was always there

when I had questions, with quick and helpful responses to my emails and even phone calls. I

have been honored to have Paul as a teacher, lab buddy, and friend.

I would like to thank my family and friends for their support throughout the year. A big hug

and thank you to Duane, Harriet, Pet, and Baby Tigger for always being there and making

my year so happy-fantastic so far. Shout out to the Hurricane Gang for how much fun they

are going to make senior spring once this thesis is finished, especially the roomies Mark,

Jake, Katie, Molly J., Will, Evan, and Molly R, honorary roommate and great friend. Finally, I

would like to thank Mom, Dad, Katie, and Barclay, for their love and support, which I

appreciate so much, no matter how far away you may be.

v

Table of Contents

Abstract ............................................................................................................................................ ii

Acknowledgements ........................................................................................................................ iv

Table of Contents ............................................................................................................................. v

Table of Figures .............................................................................................................................. vii

1. Introduction ................................................................................................................................. 1

1.1 Motivation for Fuel Cells ........................................................................................................ 1

1.2 Direct Methanol Fuel Cells ..................................................................................................... 2

1.2.1 Introduction and Operating Principles ............................................................................ 2

1.2.2 Fuel Cell Components ..................................................................................................... 3

1.2.3 Advantages and Disadvantages of the DMFC ................................................................. 4

1.2.4 Role of the membrane in the fuel cell ............................................................................ 6

1.3 Nafion® ................................................................................................................................... 6

1.3.1 Introduction .................................................................................................................... 7

1.3.2 Morphology ..................................................................................................................... 8

1.4 Importance of Mechanical Properties ................................................................................... 9

1.4.1 Introduction .................................................................................................................... 9

1.4.2 Temperature and Humidity Effects on Mechanical Properties ...................................... 9

1.5 Thesis Overview ................................................................................................................... 10

2. Background- Effects of Temperature and Solvent Activity on Tensile Creep in Nafion ............ 11

2.1 Introduction ......................................................................................................................... 11

2.2 Microstructure of Nafion ..................................................................................................... 12

2.3 Tensile Creep ........................................................................................................................ 14

2.3.1 Significance in Fuel Cells ............................................................................................... 14

2.3.2 Principles of Creep ........................................................................................................ 15

2.4 Temperature and Solvent Effects ........................................................................................ 16

2.4.1 Temperature Effects ..................................................................................................... 16

2.4.2 Solvent Activity Effects .................................................................................................. 17

2.5 Membrane Swelling ............................................................................................................. 17

2.6 Thermal History Effects ........................................................................................................ 18

2.7 Solvent Type ......................................................................................................................... 18

vi

3. Experimental Procedure ............................................................................................................ 20

3.1 Tensile Creep Apparatus ...................................................................................................... 20

3.2 Experimental Procedure ...................................................................................................... 23

3.2.1 Preparation of Materials ............................................................................................... 23

3.2.2 Tensile creep testing ..................................................................................................... 23

3.2.3 Thermal History Effects ................................................................................................. 25

3.2.4 Solvent Effects ............................................................................................................... 26

4. Experimental Results and Discussion ......................................................................................... 27

4.1 Introduction ......................................................................................................................... 27

4.2 Membrane Swelling ............................................................................................................ 27

4.2.1 Swelling Dynamics ......................................................................................................... 27

4.2.2 Equilibrium Swelling Strain ........................................................................................... 28

4.3 Nafion® Creep Response in 100% Methanol ....................................................................... 30

4.3.1 Introduction .................................................................................................................. 30

4.3.2 Results ........................................................................................................................... 30

4.3.3 Discussion...................................................................................................................... 38

4.5 Thermal History Effects ........................................................................................................ 45

4.5.1 Results ........................................................................................................................... 45

4.5.2 Discussion...................................................................................................................... 49

4.5 Solvent Effects...................................................................................................................... 50

5. Conclusion .................................................................................................................................. 56

References ..................................................................................................................................... 60

vii

Table of Figures

Figure 1.1 Schematic of a Direct Methanol Fuel Cell (not to scale) (Liu 2002)……………………….…..5

Figure 1.2 Structural formula of Nafion (Elliot, 2000)…………………………………………………………………8

Figure 2.1 Phase diagram for microphase separation in block copolymers (Benziger, 2008)……14

Figure 2.2 Plot of ideal creep response…………………………………………………………………………………….17

Figure 3.1 Schematic of Majsztrik’s creep instrument with environmental control (Majsztrik,

2007)…………………………………………………………………………………………………………………………………………23

Table 3.1 Cleaning procedure for Nafion®………..……………………………………………………………………...24

Figure 3.2 Length of Nafion® during drying procedure (Majsztrik, 2007)………………………………….25

Table 3.2 Outline of experimental procedure for tensile creep testing…………………………………….26

Figure 4.1.Membrane swelling dynamics for Nafion at 23°C at different methanol activities….29

Figure 4.2 Membrane Swelling after 12 hours equilibration at different methanol activities….30

Figure 4.3a Nafion® creep response for different methanol activities at 23oC………………………….32

Figure 4.3b Nafion® creep recovery for different methanol activities at 23oC………………………….32

Figure 4.3c Creep strain and components at 23°C as a function of water activity…………………….33

Figure 4.4a Nafion® creep response for different methanol activities at 50 oC…………………………34

Figure 4.4a Nafion® creep recovery for different methanol activities at 50 oC………………………….34

Figure 4.5a Nafion® creep response for different methanol activities at 60 oC…………………………35

Figure 4.5b Nafion® creep recovery for different methanol activities at 60 oC………………………….35

Figure 4.4c Creep strain components at 50°C as a function of water activity……………………………36

Figure 4.5c Creep strain components at 60°C as a function of water activity……………………………36

Figure 4.6 Plot of creep strain components at 23°C as a function of methanol activity……………37

Figure 4.7 Plot of creep strain components at 50°C as a function of methanol activity……………37

Figure 4.8a-f Creep strain components vs. temperature for different activities……………………….38

Figure 4.9 Nafion creep response for different water activities and temperatures (Majsztrik,

2007)…………………………………………………………………………………………………………………………………………42

Figure 4.10 Phase diagram for microphase separation in block copolymers (Benziger,

2008)…………………………………………………………………………………………………………………………………….….44

Figure 4.11 Creep strain components at 23°C as a function of methanol activity for different

drying conditions………………………………………………………………………………………………………………………47

Figure 4.12a Nafion creep response at 23 °C for different activities and drying temperatures

(dried for 3 hours)…………………………………………………….………………………………………………………………48

viii

Figure 4.12b Nafion creep recovery at 23 °C for different activities and drying temperatures

(dried for 3 hours)…………………………………….………………………………………………………………………………48

Figure 4.13a Nafion creep response at 23 °C for different activities and drying temperatures

(dried for 12 hours)………………………………………………………….……………………………………………………...49

Figure 4.13b Nafion creep recovery at 23 °C for different activities and drying temperatures

(dried for 12 hours)……………………………………………………………………………………………………………….…49

Figure 4.14 Comparison of total creep strain for the three solvent types…………………………….….52

Figure 4.15a Nafion creep response at 23 °C for 50/50 methanol/water mixtures (by mol

fraction)…………………………………………………………………………………………………………………………………….53

Figure 4.15b Nafion creep recovery at 23 °C for 50/50 methanol/water mixtures (by mol

fraction)…………………………………………………………………………………………………………………………………….53

Figure 4.16a Nafion creep response at 60°C for 50/50 methanol/water mixtures (by mol

fraction)……………………………………………………………………………………………………………………………………54

Figure 4.16b Nafion creep recovery at 60°C for 50/50 methanol/water mixtures (by mol

fraction)………………………………………………………………………………………………………………………………..….54

Figure 4.17. Nafion® creep response at 23oC for various water activities. (Courtesy of

Majsztrik)……………………………………………………………………………………………………………………………..….56

Figure 4.18. Nafion® creep response at 50oC for various water activities. (Courtesy of

Majsztrik)………………………………………………………………………………………………………………………………...56

1

1. Introduction

1.1 Motivation for Fuel Cells

As fossil fuel reserves are depleted and global warming becomes a larger concern,

alternative technologies for providing clean energy to the world are receiving increasing

attention. The Polymer Electrolyte Membrane (PEM) fuel cell will very likely play an

important role in building a renewable energy economy. PEM fuel cell applications range

from power sources for vehicles to cell phone batteries to energy storage applications. Fuel

cells are receiving increasing attention due to significant technical advances. However, in

order for the fuel cell to become commercially competitive, further developments in

technology are needed.

A fuel cell is an electrochemical device which converts chemical energy from a variety of

fuels into electrical energy. The most common type, the hydrogen fuel cell, is based on the

electrochemical reaction of hydrogen and oxygen. However, in recent years, direct

methanol fuel cells (DMFCs) have received increasing attention, especially as power sources

for various portable devices. Electronic devices today are smaller than ever, but the size of

the batteries is a roadblock to further miniaturization. DMFC’s are an ideal solution to this

problem because they can store very high energy content in a small space. Direct methanol

fuel cells are still a relatively new technology, but as they are put to more uses and become

better understood, applications may spread to other sectors, such as transportation and

energy storage.

2

Fuel cells are still expensive compared to the traditional fossil fuel sources. In order for

fuel cells to become more practical for everyday commercial use, many technical challenges

must be overcome. Durability, reliability, and performance must increase while price and

weight decrease (Majsztrik, 2007). In order to address some of these challenges, this thesis

will focus on the membrane material, an integral part of PEM fuel cells. It will investigate

the mechanical properties of the most common polymer electrolyte membrane, Nafion®, at

a range of temperatures and solvent activities.

1.2 Direct Methanol Fuel Cells

1.2.1 Introduction and Operating Principles

A fuel cell is an electrochemical energy conversion device which converts the

chemical energy from a variety of fuels into useful electrical energy. Perhaps the most

important property of a fuel cell is that unlike those of a battery, reactants and products of

a fuel cell are continuously fed to and removed. There are many types of fuel cells, including

Proton Exchange Membrane (PEM), Solid Oxide, Alkaline, Molten Carbonate, and Direct

Methanol Fuel Cells (DMFC’s). Both PEM and Direct Methanol Fuel cells utilize membranes

like Nafion®; however due to the nature of the solvents used, this thesis will focus mainly on

DMFC’s.

A DMFC is a type of PEM fuel cell in which the fuel, methanol, is not reformed, but fed

directly into the fuel cell. The methanol and water feed is internally reformed on the

catalyst layer. The hydrogen is then oxidized at the anode to produce hydrogen ions and

electrons. This complex oxidation process is shown in equation 1.1. The electrons travel

through the external circuit where they can do useful work. The hydrogen ions travel

3

through the polymer electrolyte membrane to the cathode and react with oxygen and the

electrons from the external circuit to form water, according to equation 1.2 The overall cell

reaction is thus the reaction of methanol and oxygen to produce water and carbon dioxide,

shown in Equation 1.3.

���������: �� � + ��0 → � + 3��; 3�� → 6�� + 6�� �1.1�

���������: 3

2 � + 6�� + 6�� → � + 3�� �1.2�

�� � +3

2 � + ��0 → � + 3�� �1.3�

1.2.2 Fuel Cell Components

The DMFC electrode/electrolyte system is a three layer membrane and electrode

assembly (MEA). An ionomeric membrane, usually Nafion® is sandwiched between the

anode and cathode catalyst layers. Electrodes are usually made with woven carbon cloth or

carbon paper. On the side facing the membrane, both the anode and cathode are coated

with a porous platinum-based catalyst. There are normally highly porous conductive carbon

fabric gas diffusion layers (GDLs) between the catalyst layer and the electrode. The GDLs

collect current from the catalyst layers and inhibit mass transport of reactants and products

between the electrodes and the flow fields (Liu, 2002). A good three-phase contact

between the membrane, the porous electrodes, and the gas is very important for

performance. The MEA is clamped between a pair of graphite blocks which are electrically

conducting and contact the electrodes. They contain flow channels which distribute the

reactants and products to the anode and cathode. A schematic of a typical DMFC is shown

in Figure 1.1.

4

Figure 1.1 Schematic of a Direct Methanol Fuel Cell (not to scale) (Liu 2002)

1.2.3 Advantages and Disadvantages of the DMFC

The DMFC has several important advantages over the conventional hydrogen fuel cell.

First of all, the fuel, liquid methanol, does not need to be externally reformed, but is fed

directly into the fuel cell. The system produces electric power by direct conversion of

methanol at the anode. Therefore, complicated and expensive catalytic reforming of the

fuel is not needed. This is particularly attractive for transportation applications, which rely

on bulky and often unresponsive reformer systems to convert methanol or other

hydrocarbon fuels to hydrogen (Hogarth, 1996). Also, methanol storage is much more

convenient than hydrogen storage because it does not need to be at high pressures or low

temperatures. Methanol is a liquid from -97 to 64.7 degrees Celsius at a pressure of 1 bar

5

(Lamy et.al, 2002). Another main advantage of the DMFC is that it can store high energy

content in a small space. The energy density of methanol is an order of magnitude greater

than highly compressed hydrogen and 20-30 times that of lithium-ion, the primary storage

component of the best batteries (Holladay, 2002). This makes the DMFC ideal for small

portable devices such as laptops and cell phones. However, despite high their high energy

density, the power density of DMFC’s is still significantly lower than their hydrogen

counterparts. Power densities of up to 0.3 W/cm2

have been obtained for small PEM

hydrogen fuel cells (Scholta, 2006), whereas the maximum power density for small DMFC’s

is about 50 mW/cm2 (Chang, 2002).

Despite these advantages, commercialization of the DMFC has been impeded due to its

poor performance compared with hydrogen fuel cells. Low-temperature reformation of

methanol to hydrogen and carbon dioxide requires a highly efficient catalyst. Studies have

shown that only expensive platinum-based materials show reasonable activity and the

required stability (Hogarth, 1996), and the DMFC requires a larger quantity of this platinum

catalyst than conventional PEMFC’s. A key problem with DMFC’s is the high permeation of

methanol through the membrane from the anode to the cathode, a process known as

methanol crossover. This results in lower efficiency and sluggish dynamic behavior.

Furthermore, a DMFC can only produce limited power, and the output power of the cell is

dependent on temperature and the quantity of fuel in the fuel cell. Recent work by

Majsztrik and Satterfield has shown that the mechanical and transport properties of Nafion®

are affected by water. A similar effect on mechanical properties may occur with methanol,

and this will affect the occurrence of methanol crossover.

6

PEM electrolyte materials have extended the operation temperature of the DMFC

beyond those attainable with traditional liquid electrolytes, which has lead to major

improvements in performance in recent years. However, reliability, performance and cost

need to improve before the implementation of the DMFC can become widespread.

1.2.4 Role of the membrane in the fuel cell

The membrane is a central component of the PEM fuel cell. The purpose of the

membrane is to physically separate the fuel from the oxidant, while allowing protons to

move across the membrane but not electrons. Thus, it must transport protons with little

resistance and be a poor conductor of electrons. When the Nafion® is dry, its acid groups

are tightly bound, like a salt, and don’t allow any proton conduction. A solvent such as water

or methanol is essential for proton conductivity. For example, water has been found to

increase membrane conductivity by several orders of magnitude going from dry to fully

saturated (Yang, et. al., 2004).

The material must also be mechanically and chemically stable under the range of

operating conditions of the fuel cell environment, such as temperature, pressure, and

solvent activity. This means that its properties should not change too much under different

conditions. There are very few materials that do this well. Nafion, the most commonly used

membrane in PEM and Direct Methanol Fuel Cells, doesn’t accomplish all of these criteria

well, but it is the only ionomer with a lifetime greater than 1000 hours in a fuel cell

environment. Therefore, further research is needed to determine how the properties of

Nafion® as a fuel cell membrane can be improved.

1.3 Nafion®

7

1.3.1 Introduction

The membrane studied in this work is Nafion®, which is a sulfonated

tetrafluorethylene copolymer, the first of a class called perfluoronated ionomers. It was

developed by DuPont in the late 1960’s for use in the chlor-alkali process. Since then,

Nafion® has become the most commonly used PEM fuel cell membrane material due to

favorable conductivity, chemical and mechanical stability, and mechanical toughness. The

molecular structure of Nafion® is shown below. The value of m is usually one, so that the

value of n determines the ratio of polar to non-polar material in the compound.

Figure 1.2 Structural formula of $afion (Elliot, 2000)

The structure of Nafion® is shown in Figure 1.2. Nafion® consists of a fluorinated-

carbon backbone with pefluoroether (PFE) side chains ending in sulfonic acid groups (Elliot,

2000). The backbone is tetrafluoroethylene (TFE) or Teflon-PTFE®, and gives Nafion® its

strength and toughness. The sulfonic acid side groups are arranged at intervals along the

backbone. These give Nafion® its chemical and mechanical stability. Conventionally,

membranes are named according to their equivalent weight (EW) and thickness. For

example, Nafion® 1110, which is used in our lab, has an EW of 1100 g/mol-SO3 and a

thickness of 0.010”.

8

1.3.2 Morphology

The detailed characterization of Nafion is important for the development of all its

technical applications. Although Nafion’s® morphology has been the subject of extensive

research since the 1970’s, the exact structure of Nafion® is still not known. Several models

have been proposed which were summarized by Roberson, including the Eisenberg Model

of Hydrocarbon Ionomers, the Gierke Cluster Network Model, the Mauritz-Hopfinger

Model, and the Yeager Three Phase Model (Robertson, 1994). The most widely adopted

model is the one first proposed by Gierke, which assumes that the hydrophilic groups

aggregate into clusters which grow in size with increasing hydration (Benziger, 2008).

All models agree on the fact that that Nafion® phase separates into discrete

hydrophobic and hydrophilic regions. The hydrophobic region is the fluorocarbon backbone,

and the hydrophilic region is the PFE side chains terminating in sulfonic acid groups. Small

angle X-ray scattering (SAXS) and neutron scattering experiments have clearly shown that

ionic clustering is present in Nafion®, in which the ionic groups tend to form tightly packed

regions called clusters as a result of electrostatic interactions (Yeager et. al, 1982). However,

the details of the arrangement of the clusters are not known exactly.

The information on the structure of Nafion® depending on water content was

summarized nicely in the recent work of Mauritz and Moore (Mauritz, 2004): The

morphology of the Nafion® reorganizes as the dry membrane is swollen with water. The dry

state consists of isolated spherical ionic clusters with diameters of around 1.5 nm dispersed

in the perfluorinated matrix. When the ionomer takes up water, the clusters start to hydrate

and their diameters increase up to 4 nm. They form “nano-pools” of water that are

9

surrounded by ionic sulfate groups that are connected by tiny channels. Very little work has

been done regarding Nafion’s® structure with methanol uptake.

1.4 Importance of Mechanical Properties

1.4.1 Introduction

Studies of polymer electrolyte membrane (PEM) fuel cell dynamics using Nafion®

membranes have demonstrated the importance of membrane mechanical properties to

performance and longevity of the cell (Satterfield 2007). Thus, an understanding of the

mechanical properties of membranes in PEM fuel cells is essential to improving the

efficiency, performance, and cost of the fuel cell. One of the most important mechanical

properties studied in fuel cell membranes in tensile creep. In a Direct Methanol Fuel Cell,

creep results in mechanical problems that increase the incidence of methanol crossover, the

leading failure mechanism in DMFC’s. Additionally, a material’s mechanical properties are

strongly dependent the temperature and solvent activity of their environment.

1.4.2 Temperature and Humidity Effects on Mechanical Properties

One of the main faults of Nafion® as a fuel cell membrane material is that its

mechanical properties are strongly dependent on temperature and hydration. Due to the

nature of the reactants and products, fuel cells operate at elevated humidities (or solvent

activities). Furthermore, increasing the operating temperature of fuel cells is desirable for

several reasons. However, common fuel cell membranes including Nafion® perform poorly

under these conditions and degrade more quickly. Improving membrane performance over

a variety of conditions simplifies operation controls and improves overall performance.

10

Because of this, an understanding of the mechanical properties under a range of conditions

is very important, although very little work has been done in this area.

1.5 Thesis Overview

This thesis will present studies on the mechanical properties of Nafion®, specifically with

respect to viscoelastic creep and membrane swelling. It will investigate the effects of both

methanol and water on the viscoelastic response of Nafion® membranes. As fuel cells are

operated at elevated temperatures and humidity, it is important to take environmental

conditions into account when studying membrane mechanical properties. Thus, effects due

to methanol in the presence and absence of water will be studied at different temperatures

and solvent activities. A unique apparatus built by Paul Majsztrik of Princeton University

allows for an analysis of viscoelastic response at carefully controlled temperature and

humidity, an area in which very little work has been possible in the past due to limitations in

experimental equipment. Additionally, the effect of drying time and temperature on creep

results will be examined. Finally, membrane swelling dynamics and equilibrium membrane

swelling will be investigated, as membrane swelling is closely related to creep in fuel cell

membranes. The primary goal of these experiments is to gain a better understanding of

Nafion’s morphology, a subject that is still debated in the field despite extensive studies

performed on the polymer. Thus, experimental results will be attempted to be explained

based on microstructure and interactions at a molecular level. A secondary goal is to

determine pretreatment and environmental conditions for Nafion that maximize its

mechanical performance.

11

2. Background- Effects of Temperature and

Solvent Activity on Tensile Creep in $afion

2.1 Introduction

Nafion® is a viscoelastic material, which means that it responds to stress in a time-

dependent manner. The membrane of a PEM fuel cell is subject to various stresses, for

example, strain from changing levels of hydration as the membrane takes up water and

swells. The clamping of the membrane between flow plates also puts stress on the

membrane. These stresses will cause a membrane such as Nafion® to flow and respond

dynamically to changes in hydration and stress. An accurate knowledge of the mechanical

properties of the membrane is crucial to increased understanding of its failure mechanisms

and therefore can lead to improved design and performance of the fuel cell. One of the

most commonly studied mechanical properties and the main subject of this thesis is tensile

creep. Creep is the term used to describe the time dependent deformation of a material in

order to relieve applied stresses.

The mechanical properties of Nafion have been the subject of extensive studies since

the polymer’s development in the 1960’s. However, due to constraints in available

equipment, few studies have been done on the mechanical properties of Nafion under a

wide range of temperatures and humidities. Majsztrik’s creep apparatus with environmental

control has resulted in extensive studies of Nafion’s® viscoelastic response over a range of

temperatures and water activities (Majsztrik, 2007). To the author’s knowledge, no such

studies have been done using methanol. The goal of this thesis is to present results related

to the viscoelastic response of Nafion over a range of temperatures and methanol activities.

The major areas of study are as follows:

12

1) Equilibrium Membrane Swelling

2) Effects of solvent activity and temperature on creep response

3) Thermal history effects

4) Solvent effects

2.2 Microstructure of Nafion

Dependence of mechanical properties of Nafion on both temperature and solvent

activity suggest that there are structural changes in Nafion which result in changes in

mechanical properties. Therefore, an understanding of the microstructure of Nafion is

extremely important for predicting the effect of the environmental conditions on

mechanical properties of Nafion®.

Microphase separation in block copolymers is a well-known phenomenon, however,

it has been suggested that phase separation also occurs with random copolymers, such as

Nafion® (Benziger, 2008). The basic principle behind microphase separation is that phases

restructure themselves so as to minimize free energy. In Nafion®, different phases form to

try to minimize the interaction between the hydrophobic and hydrophilic phases. Nafion has

the unique property that its two components exhibit very different hydrophilicities. The TFE

backbone is very hydrophobic while the strong hydrogen bonding capacity of the sulfonic

acid groups makes them very hydrophilic.

It is possible to continuously vary the volume fraction of the hydrophilic phase by

controlling the activity of a hydrogen bonding solute surrounding the polymer (Benziger,

2008). It has been proposed that a change in the volume fraction of the hydrophilic vs. the

hydrophobic phase due to absorbed solute results in microphase evolution from dispersed,

to cluster, to cylindrical phases with increasing solute activity (Benziger, 2008). This

13

microphase evolution also depends on temperature; an increase in temperature decreases

the interaction parameter, generally the Flory parameter χ. It is the combined effects of

temperature and solute activity which lead to a restructuring of the hydrophilic microphase,

thus a change in mechanical properties. Thus, microphase separation is a function of the

volume fraction of the hydrophilic phase φ, the energy difference between monomers χ,

and thus the temperature T. Figure 2.1 is a phase diagram developed for block-copolymers

developed by Matsen and Bates (1996) and modified by Benziger (2008). Since Nafion is a

random copolymer, we do not expect phase transitions to be exactly the same as block co-

polymers, but we do know that the driving forces for microphase separations should be very

similar, and thus we expect similar qualitative behavior.

Figure 2.1 Phase diagram for microphase separation in block copolymers (Benziger, 2008)

14

2.3 Tensile Creep

2.3.1 Significance in Fuel Cells

In a fuel cell membrane, creep can result in membrane thinning, the formation of

pinholes, and the development of contact problems between the membrane and the

electrode, all of which can lead to the failure of the PEM fuel cell (Majsztrik, 2007). Pinhole

formation is an especially large problem for DMFC’s because it increases the occurrence of

methanol crossover, the leading failure mode DMFC’s. In addition, creep increases the area

methanol permeation, which causes more methanol crossover to occur. Methanol crossover

is a process by which methanol moves from the anode to the cathode through the

membrane. It leads to power loss due to i) oxidation of methanol at the cathode, causing

unwanted consumption of oxygen, ii) poisoning of the cathode by CO, an intermediate

product of methanol oxidation, and iii) excessive water buildup by the cathode produced by

methanol oxidation (Lin et. al., 2006) Methanol crossover not only lowers fuel utilization

efficiency, but also further increases the size and complexity of the fuel cells in order to

manage the excess heat and water produced as methanol is oxidized on the air side of the

fuel cell (Jiang, 2004). Therefore, if creep strain in Nafion could be reduced by varying

pretreatment or environmental conditions, fuel cell performance could be improved.

Reliability of PEM fuel cell power systems is mostly dependent of the durability of

the Membrane Electrode Assembly (MEA). Thus, it is extremely important to understand

the mechanical properties of the Nafion® membrane, as it leads to increased understanding

of failure mechanisms of the Membrane Electrode Assembly. Furthermore, accurately

knowing the mechanical properties of Nafion® at a range of temperatures and solvent

activities leads to better modeling of fuel cell performance under many different possible

15

environmental conditions. In addition, the dynamics of fuel cell power response are partially

dependent on mechanical properties (Satterfield et. al., 2006).

2.3.2 Principles of Creep

Creep is the term used to describe the time dependent deformation of a material in

order to relieve applied stresses. It is an excellent technique for characterizing the

viscoelastic response of a material. Creep is measured simply by applying a constant stress σ

to the material and recording the resulting strain ε which increases with time. Stress and

strain are defined by the following equations:

Stress: σ=F/A

Strain: ε= λ�λ

λ

where F is the applied force, A is the cross sectional area of the material, and λ0 and λ are

the initial and final lengths of the material, respectively.

There are three components of an ideal creep response. Instantaneous elastic

creep (εe), occurs immediately and is completely recoverable. This is simply due to the

stretching and bending of bonds between chains. Following this, strain increases with time

at a decreasing rate in the delayed elastic (εd) creep phase. This component is also

completely recoverable, and is the result of these chains uncoiling. Finally, viscous flow (εv)

occurs due to chain slipping, which is irrecoverable. The contribution of these different

components can be separated by allowing a sample to creep under stress and then undergo

recovery under zero stress. The three components of ideal creep response are shown in

Figure 2.2.

16

Figure 2.2 Plot of ideal creep response (Majsztrik, 2007)

2.4 Temperature and Solvent Effects

2.4.1 Temperature Effects

Increasing the operating temperature of fuel cells is desirable because it decreases

CO poisoning of the catalyst. CO and H2 compete to absorb on the platinum based catalyst.

Below 100°C, if the concentration in CO is higher than 1-10 ppm, CO will win this

competition, which results in a decline in performance. As the temperature of the fuel cell

increases, more and more CO can be tolerated. Higher operating temperatures also make

the rejection of waste heat easier. However, common fuel cell membranes including

Nafion® perform poorly at elevated temperature and degrade more quickly.

As would be expected, the mechanical properties of Nafion® are strongly dependent

on temperature, due to the fact that the bonding strength between polymer chains

decreases with increasing temperature. As temperature increases, the thermal energy of

Nafion’s® molecules eventually becomes greater than that of the polymer’s secondary

bonds, and chains are able to move past one another when a force is applied. Secondary

17

bonds in Nafion® include hydrogen bond cross-linking between the sulfonic acid groups and

Van der Waals interactions between main chains.

2.4.2 Solvent Activity Effects

Solvent activity, of both methanol and water, strongly affects Nafion’s® mechanical

properties. Solvent activity is closely related to solvent mass uptake, as increased solvent

activity should result in increased uptake. Several studies, including those of Tsai and Hwang

(2007) and Majsztrik (2007) have shown that solvent uptake strongly affects the mechanical

properties of the polymer. Other studies have shown that solvent uptake results in changes

in the size, shape and number of acid clusters of Nafion® (Hsu et. al, 1983). Previous studies

have been restricted to measuring properties as a function only of temperature or a very

limited range of hydrations. This thesis will investigate solvent activities ranging from 0 to

~1 for both 100% methanol and a 50/50 methanol/water mixtures at different activities.

Extensive studies for 0-100% relative humidity have already been performed by Majsztrik

(Majsztrik, 2007). Some of the results of these studies will be included in this thesis for

comparative purposes.

2.5 Membrane Swelling

Exposing Nafion® to methanol vapor results in uptake of methanol by the

polymer, referred to as equilibrium mass uptake. Equilibrium mass uptake is a strong

function of the methanol activity, and also depends weakly on temperature. It is what

causes Nafion® to swell, among other things. Swelling is very important in the context of

fuel cells because since the membrane is clamped between two plates, swelling due to mass

uptake causes stresses in the membrane, which result in strain. This swelling strain is a

18

result of polymer reorganization as methanol content changes. Equilibrium mass uptake is

also important in fuel cells because proton conductivity is dependent on solvent activity

(Duplessix et. al, 1979).

2.6 Thermal History Effects

Studies have shown that Nafion’s® performance and intrinsic properties are

dependent not only on its chemical identity but also on its thermal history, for example,

drying and exposure to high temperature (Hensley et.al., 2007). Therefore, when using

Nafion® for fuel cells or any application, one must take careful account of thermal history.

Ideally, a pretreatment procedure would be developed for Nafion® that would result in

optimal performance of the membrane. Drying is an important part of pretreatment, and

can have a significant effect on Nafion’s® mechanical properties. Temperature changes may

cause a change in morphology, which would change the creep response of the polymer.

Changes in morphology may also be dependent on the amount of time exposed to drying

conditions.

2.7 Solvent Type

There are two relevant solvents in a DMFC: methanol and water. Since methanol is

fed directly into the cell as fuel, clearly the membrane will be exposed to methanol. The

reaction at the cathode produces water (see equation 1.1), but water is also required at the

anode side of the DMFC in order to maintain the reaction. Conventional DMFC systems have

an active water management system with a water recovery pump and a recirculation pump

to achieve this. Thus, the membrane is exposed to both methanol and water during fuel cell

19

operation. Nafion’s® ionic groups can absorb water and methanol, which alters the bonding

between polymer chains and therefore also the mechanical properties (Majsztrik, 2007).

Solvents that were a mixture of methanol and water were investigated to determine

the synergistic effects of methanol and water on Nafion® creep response, as well as to

compare creep response when the membrane is exposed to water, methanol, or a

combination thereof. This allows one to gain a further understanding of the solvent

interactions with Nafion® that give rise to its mechanical properties. Clearly, the solvent

type will affect creep response due to differing interactions between different solvents and

the Nafion®, as well as possible differences is mass uptake of the two solvents by Nafion®.

20

3. Experimental Procedure

3.1 Tensile Creep Apparatus

Commercially available instruments for testing mechanical properties of materials

offer little control over solvent activity over a range of temperatures. The apparatus (patent

pending) used in this experiment was designed by Paul Majsztrik of Princeton University

specifically to measure creep response of Nafion® under a controlled environment of

temperature and water activity. It was a simply matter to modify the experimental

procedure already developed by Majsztrik in order to incorporate controlled methanol

activity. Figure 3.1 is a schematic of the creep instrument with environmental control.

The environmental creep apparatus is described in detail elsewhere (Majsztrik,

2007), but will be explained briefly here. The sample is clamped between a stationary upper

clamp and a moveable lower clamp. A hanging 150.4 gram weight is used to apply uniaxial

stress to the sample through a rod connected to the bottom clamp. The clamps allow for a

maximum strain of 2.25 for a typical 1” sample. Strain is measured with a Linear Variable

Displacement Transducer (LVDT) (Macro Sensors, HSAR 750-2000) by monitoring the

position of one section of the rod connecting the hanging mass. This provides a

measurement of strain without contact. A universal joint prevents the transmission of

torque due to misalignment, while PFTE guides prevent the rod from swinging. The different

components of the instruments are mounted on a vertical aluminum plate with leveling

feet. A sliding stage is used to apply and relieve the mass from the sample.

An environmental chamber consisting of an insulated outer shell and a humidified

inner chamber surrounds the sample. Both chambers sit on a stationary base which also

21

serves as a mount for sensors and other components needed to control the temperature

and solvent activity of the environment. The insulated outer shell consists of this base and a

removable upper box insulated with a fiberglass insulation board. The humidity chamber is

a Pyrex reaction vessel which sits on a stainless steel base and surrounds the sample.

An isothermal environment is created around the humidity chamber using a finned

heater and a fan together with a PID temperature controller. A temperature range of 25 -

250°C is possible. The apparatus also allows for careful control of solvent activity. The

activity of a component of an ideal gas mixture is defined as the partial pressure of that

component divided by the vapor pressure of the component at the given conditions.

However, due to limitations in the experimental apparatus, these partial pressures cannot

be directly measured. Therefore, activity is controlled by using mass flow controllers to vary

the flow rates of a dry nitrogen stream and a nitrogen stream which passes through the

bubbler to become saturated with methanol. Methanol activity is then calculated as the

flow rate of the nitrogen stream saturated with methanol divided by the flow rate of the

saturated and the dry stream. This calculation is given in equation 3.1. The key assumption

made is that the nitrogen stream is completely saturated with methanol. Dry nitrogen gas is

used when zero activity is required. The bubbler is contained within the outer

environmental chamber to ensure constant gas temperature throughout.

! =#$%&'(%&)* +� [

-.-��

]

#*(0 +� [-.-��

] + #$%&'(%&)* +� [-.-��

] �3.1�

A Labview program is used to record the length of the sample over time, from which

strain can be calculated. Labview provides a straight-forward interface between data

22

acquisition and the computer. Data is acquired every second for the first 20 minutes of a

creep experiment, and once every minute thereafter. Similar data acquisition methods are

used for creep recovery.

Figure 3.1 Schematic of Majsztrik’s creep instrument with environmental control

(Majsztrik, 2007)

23

3.2 Experimental Procedure

3.2.1 Preparation of Materials

All tests were performed on extruded Nafion® N1110 film, which has an equivalent

weight of 1,100 g/mol-SO3 and a dry thickness of 0.0010”. Before they can be used in the

experiment, extruded Nafion® films must be cleaned and cut. Nafion® films were treated

following a standard cleaning procedure developed by Paul Majsztrik, outlined in Table 3.1.

After being cleaned, the film is cut into strips of uniform width using a metal template and

an Exacto™ knife. The clean Nafion® sample was then mounted in the clamps of the creep

instrument using a mounting jig. The jig served to align the sample and hold it in place while

it was being clamped.

1. Boil 1 hour in 3% hydrogen peroxide solution

2. Boil 1 hour in distilled/deionized water

3. Boil 1 hour in 0.5 M sulfuric acid

4. Boil 1 hour in deionized water twice.

5. Dry clean polymer flat on lab bench by placing between sheets of filter paper under

a relatively heavy weight (Several books, for example) for 48 hours.

Table 3.1 Cleaning procedure for $afion®

3.2.2 Tensile creep testing

Tensile creep tests were performed on Nafion® with different methanol activities

and over a range of temperatures. Runs were done at 23, 50, and 60°C at activities 0, 0.01,

0.1, 0.35, 0.65, and ~1. In order to compare runs in a meaningful manner, a strict protocol

for membrane testing needs to be developed. Sample history (thermal and hydration),

applied stress, and sample dimensions are all important factors and must not be varied

between runs. The sample must be dried prior to the experiment to remove as much water

as possible from the sample. Additionally, the length of time to establish equilibrium

between sample and surroundings

performed in order to determine appropriate times for drying, equilibration, creep, and

creep recovery.

Nafion® samples were prepared according the

clamped into the creep inst

least three hours by running dry nitrogen at

Sample length during drying is plotted i

Nafion® shrinks as it dries.

decreases (Majsztrik, 2007)

probably due to creep caused by the pressure exerted by the clamping jaws onto the

sample. Visual inspection of the samples reveals that they are thinned and widened where

clamping occurs.

Figure 3.2 Length of $afion®

between sample and surroundings must be established. Preliminary experiments were

performed in order to determine appropriate times for drying, equilibration, creep, and

samples were prepared according the procedure given in Section 3.2.1

clamped into the creep instrument using a mounting jig. The sample was then dried for at

least three hours by running dry nitrogen at 50°C continuously through the chamber.

Sample length during drying is plotted in Figure 3.2. The length initially decreases because

as it dries. However as Nafion® is heated and dried, its resistance to creep

, 2007). Thus, the increase in length following the initial shrinking is

creep caused by the pressure exerted by the clamping jaws onto the

ple. Visual inspection of the samples reveals that they are thinned and widened where

f $afion® during drying procedure (Majsztrik, 2007)

24

Preliminary experiments were

performed in order to determine appropriate times for drying, equilibration, creep, and

procedure given in Section 3.2.1 and

The sample was then dried for at

continuously through the chamber.

. The length initially decreases because

is heated and dried, its resistance to creep

he increase in length following the initial shrinking is

creep caused by the pressure exerted by the clamping jaws onto the

ple. Visual inspection of the samples reveals that they are thinned and widened where

25

After drying, the sample is allowed to reach an equilibrium length while at the

desired temperature and methanol activity. The temperature of the environmental chamber

was set to the test temperature, and methanol was introduced using the bubbler as

described in Section 3.1. Methanol and nitrogen flows are set in order to obtain

predetermined methanol activity. It was established that 12 hours was sufficient time for

equilibration to occur.

Once equilibrium is established, creep measurements can begin. The sample is

subjected to a constant force produced by a suspended weight of 150.4 grams. The sample

is allowed to creep for exactly one hour, after which the weight is relieved and creep

recovery begins to occur. It was determined that three hours was sufficient time for all

creep recovery to occur. During the entire process, sample length was monitored and

recorded as a function of time. The overall experimental procedure is outlined in Table 3.2

Step Flow parameters Time

Drying Pure nitrogen at 50oC 3 hours

Heating/cooling (if necessary) Pure nitrogen at desired temp. 1 hour

Equilibration Nitrogen/methanol at

predetermined activity

12 hours

Creep Nitrogen/methanol at

predetermined activity

1 hour

Creep Recovery Nitrogen/methanol at

predetermined activity

3 hours

Table 3.2 Outline of experimental procedure for tensile creep testing

3.2.3 Thermal History Effects

The effects of thermal history on the viscoelastic response of Nafion® were

investigated by varying the drying time and temperature from that outlined in the above

procedure. Creep runs were carried out at room temperature after drying at both 50 and

100°C for 3 and 12 hours. This was done at three activities: 0, 0.1, and 0.65. Since the boiling

26

point of methanol is 64.7°C13

, the methanol must be removed from the chamber during the

drying step at 100°C and reintroduced during the equilibration step.

3.2.4 Solvent Effects

The synergistic effects of a water-methanol mixture were investigated at 23°C. In

addition to a 100% methanol solvent, the experiment was repeated using a solvent of 50%

methanol and 50% water. Experiments using 100% water have already been performed by

Paul Majsztrik, and were not repeated. However, data from these experiments, courtesy of

Majsztrik, is presented in the Results section for comparative purposes. The procedure in

Section 2.3.2 for tensile creep testing was followed, with the methanol in the bubbler being

replaced with a methanol-water mixture of different compositions. Runs were done at 23

and 50°C for activities 0.01, 0.1, 0.35, 0.65, and ~1.

27

4. Experimental Results and Discussion

4.1 Introduction

Creep and creep recovery were measured at over a range of temperatures for

various methanol activities. Solvents of both 100% methanol and a 50/50 methanol-water

mixture were investigated. Summary data from 100% water, courtesy of Paul Majsztrik, is

also presented here for comparative purposes. The effect of thermal history on creep and

creep recovery was also investigated. The results from the various studies are presented in

this section divided by experimental technique.

4.2 Membrane Swelling

4.2.1 Swelling Dynamics

Sample strain was monitored throughout the 12 hour equilibration period in order

to gain a better understanding of the swelling dynamics of Nafion® in methanol. The plot in

Figure 4.1 shows sample strain over time as the dry sample is exposed to methanol vapor at

activities .01, .10, .35, .65, and ~1 at 23°C. As expected, equilibrium swelling strain increases

monotonically with methanol activity. The next thing we notice is the effects of the

methanol vapor are felt much later in the environmental chamber for lower activities. It

takes over 14,000 seconds for the strain at activity .01 to start to increase from zero,

whereas for activity .35, it takes only a few hundred seconds. This is probably due to the

slow diffusion of methanol through the polymer at low concentration gradients. The initial

slope for activity .01 is significantly less than the slopes at higher activities. This means that

time for swelling strain to reach its equilibrium value increases significantly at very low

activities. The initial slopes for the higher activities are very similar. This is probably due to

28

interfacial mass transport resistance being the limiting step to water uptake (Majzstrik,

2007). This is based on Satterfield’s finding that mass transport limits mass uptake for dry

Nafion® exposed to water vapor with activity approaching 1.0 (Satterfield, 2007). It is

reasonable to infer that one might see a similar result with methanol.

Figure 4.1.Membrane swelling dynamics for $afion at 23°C at different methanol activities

4.2.2 Equilibrium Swelling Strain

Membrane swelling in equilibrium with different solvents was investigated in order to

determine the behavior of Nafion® in various methanol activities in the absence of any

outside stress. This was done for all creep tests performed in this thesis by recording the

length of the sample before and after the 12 hour equilibration period and calculating the

percentage change in length. The results for methanol at 23, 50 and 60 °C are given in

Figure 4.2. As expected, equilibrium swelling increases monotonically with methanol

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 10000 20000 30000 40000 50000

Str

ain

(m

m/

mm

)

Time (s)

Activity .01

Activity .10

Activity .35

Activity .65

Activity ~1

29

activity. Little dependence was seen on temperature, which is consistent with the results of

Majsztrik for equilibrium swelling in water. It is interesting that samples equilibrating at

60°C consistently seemed to swell less than those equilibrating at 50°C, although much

more creep occurred at 60°C. This could be due to a lesser degree of methanol saturation of

nitrogen at 60°C, so that the true activities are slightly lower than stated.

Exposing Nafion to methanol vapor results in methanol uptake by the membrane. The

increasing swelling strain with methanol activity is due to increased methanol uptake by the

polymer. Mass uptake increases the size of the acid clusters as hydrogen bonds are formed

between the polar methanol and the hydrophilic sulfonic acid groups. Dimensional swelling

is necessary for the Nafion to accommodate the larger solvated acid clusters. Equilibrium

mass uptake is a strong function of activity, but only depends weakly on temperature. This

was confirmed by the fact that membrane swelling increased consistently with activity, but

only a small correlation was seen with temperature.

0

2

4

6

8

10

12

14

.01 .10 .35 .65 ~1

% C

ha

ng

e i

n S

am

ple

Le

ng

th

Methanol Activity

23°C

50°C

60°C

Figure 4.2 Membrane welling after 12 hours of equilibration in different methanol activities

30

4.3 Nafion® Creep Response in 100% Methanol

4.3.1 Introduction

Tensile creep was measured at 23,50 and 60°C for activities 0, 0.01, 0.10, 0.35, 0.65,

and ~1. Nitrogen and methanol were supplied through a bubbler at different flow rates to

obtain these different activities. Sample width and thickness before creep were 0.25” and

0.01” respectively, and the initial gauge length was 1.00”. The applied force for all runs was

1.48 Newtons. The repeatability of the creep apparatus was initially tested by repeating

runs. Results were found to be repeatable with an error of around 5%. Additionally, some

runs were repeated to confirm specific results.

4.3.2 Results

Results for creep response at room temperature, around 23oC, are given in Figures

4.3a and 4.3b. Creep response increases with activity. The extremes of this result are seen in

runs with activity around 0 and 1. Almost no creep occurs for activity ~0, whereas for

activity ~1, strain rises above 0.25. Nafion creep can be analyzed in more detail by breaking

total creep strain down into its components. Total creep strain, instantaneous elastic creep,

delayed elastic creep, and viscous losses were determined for every run. An analysis of

creep components at 23 o

C is found in Figure 4.3c.

31

0

0.05

0.1

0.15

0.2

0.25

0.3

0 500 1000 1500 2000 2500 3000 3500 4000

Str

ain

(m

m/m

m)

Time (s)

activity 0

activity .01

activity .10

activity .35

activity .65

activity 1

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0 2000 4000 6000 8000 10000 12000

Str

ain

(m

m/m

m)

Time (s)

activity 0

activity .01

activity .10

activity .35

activity .65

activity 1

Figure 4.3b $afion® creep recovery for different methanol activities at 23oC.

Figure 4.3a $afion® creep response for different methanol activities at 23oC.

32

0

0.05

0.1

0.15

0.2

0.25

0 0.01 0.1 0.35 0.65 1

Str

ain

(m

m/

mm

)

Methanol Activity

Total Strain

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.01 0.1 0.35 0.65 1

Str

ain

(m

m/

mm

)

Methanol Activity

Instantaneous Elastic Strain

0

0.02

0.04

0.06

0.08

0.1

0 0.01 0.1 0.35 0.65 1

Str

ain

(m

m/

mm

)

Methanol Activity

Delayed Elastic Strain

0

0.02

0.04

0.06

0.08

0.1

0 0.01 0.1 0.35 0.65 1

Str

ain

(m

m/

mm

)

Methanol Activity

Viscous Loss

The experiment was repeated at elevated temperatures of 50oC and 60

oC. The

boiling point of methanol is 64.7oC, so the experiment cannot be operated too close to this

temperature. The results for Nafion® creep and creep recovery at 50oC are given in Figures

4.4a and 4.4b, respectively. Again, the general trend is that creep increases with increased

activity. Interestingly, the run for activity 0.10 did not follow this trend at 50 and 60 o

C, and

showed less creep strain that activity 0.01. At 50 o

C, creep strain increased from activity 0 to

0.01, decreased at activity 0.10, and increased again up until activity 1. A similar result was

seen at 60oC, except here creep strain for activity 0.10 was even less than that for activity 0.

This indicates that there is some type of transition that occurs around activity .10. Results

for 60 o

C are shown in Figures 4.5a through 4.5c.

Figure 4.3c Creep strain and components at 23°C as a function of water activity, associated with runs shown in Figure

4.3a and 4.3b

33

0

0.1

0.2

0.3

0.4

0.5

0.6

0 2000 4000 6000 8000 10000 12000

Str

ain

(m

m/m

m)

Time (s)

Activity 0

Activity .01

Activity .10

Activity .35

Activity .65

Activity ~1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 500 1000 1500 2000 2500 3000 3500 4000

Str

ain

(m

m/m

m)

Time (s)

Activity 0

Activity .01

Activity .10

Activity .35

Activity .65

Activity ~1

Figure 4.4a $afion® creep response for different methanol activities at 50 oC

Figure 4.4a $afion® creep recovery for different methanol activities at 50 oC

34

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 500 1000 1500 2000 2500 3000 3500 4000

Str

ain

(m

m/

mm

)

Time (s)

activity 0

activity .01

activity .1

activity .35

activity .65

activity ~1

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 2000 4000 6000 8000 10000 12000

Str

ain

(m

m/

mm

)

Time (s)

activity 0

activity .01

activity .1

activity .35

activity .65

activity ~1

Figure 4.5a $afion® creep response for different methanol activities at 60 oC

Figure 4.5b $afion® creep recovery for different methanol activities at 60 oC

35

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.01 0.1 0.35 0.65 ~1

Str

ain

(m

m/

mm

)

Methanol Activity

Total Strain

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 0.01 0.1 0.35 0.65 ~1

Str

ain

(m

m/

mm

)

Methanol Activity

Instantaneous Elastic Strain

0

0.05

0.1

0.15

0.2

0.25

0.3

0 0.01 0.1 0.35 0.65 ~1

Str

ain

(m

m/

mm

)

Methanol Activity

Delayed Elastic Strain

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 0.01 0.1 0.35 0.65 ~1S

tra

in (

mm

/m

m)

Methanol Activity

Viscous Loss

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.01 0.1 0.35 0.65 ~1

Str

ain

(m

m/

mm

)

Methanol Activity

Total Strain

0

0.05

0.1

0.15

0.2

0.25

0 0.01 0.1 0.35 0.65 ~1

Str

ain

(m

m/

mm

)

Methanol Activity

Instantaneous Elastic Strain

0

0.1

0.2

0.3

0.4

0.5

0 0.01 0.1 0.35 0.65 ~1

Str

ain

(m

m/

mm

)

Methanol Activity

Delayed Elastic Strain

0

0.2

0.4

0.6

0.8

1

0 0.01 0.1 0.35 0.65 ~1

Str

ain

(m

m/

mm

)

Methanol Activity

Viscous Loss

Figure 4.5c Creep strain components at 60°C as a function of water activity, associated with runs shown

in Figure 4.5a and 4.5b

Figure 4.4c Creep strain components at 50°C as a function of water activity, associated with runs

shown in Figure 4.4a and 4.4b

36

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.01 0.1 0.35 0.65 1

Str

ain

(in

/in

)

Activity

Total

Instantaneous Elastic

Delayed Elastic

Viscous Loss

0

0.05

0.1

0.15

0.2

0.25

0 0.01 0.1 0.35 0.65 1

Str

ain

(in

/in

)

Activity

Total Creep

Instantaneous Elastic

Delayed Elastic

Viscous Flow

Additional information can be gained by examining the different creep components

separately. Figures 4.6 and 4.7 show a plot of creep strain components at 25°C and 50°C

respectively, versus activity. Results for 60°C were very similar to those at 50°C. As expected

given the results discussed above, in general all three creep components, εe, εd, and εv,

increase with activity. At higher temperatures, creep components go through a maximum at

activity .01, decrease significantly at activity .10, and increases again at higher activities.

Figure 4.6 Plot of creep strain components at 23°C as a function of methanol activity,

associated with runs shown in Figure 4.3

Figure 4.7 Plot of creep strain components at 50°C as a function of methanol activity,

associated with runs shown in Figure 4.4

37

0

0.05

0.1

0.15

0.2

0.25

0.3

23 50 60

Str

ain

(in

/in

)

Temperature (C)

Total

Instantaneous Elastic

Delayed Elastic

Viscous Flow

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

23 50 60

Str

ain

(in

/in

)

Temperature (C)

Total

Instantaneous Elastic

Delayed Elastic

Viscous Flow

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

23 50 60

Str

ain

(in

/in

)

Temperature (C)

Total

Instantaneous Elastic

Delayed Elastic

Viscous Flow

0

0.1

0.2

0.3

0.4

0.5

0.6

23 50 60

Str

ain

(in

/in

)

Temperature (C)

Total

Instantaneous Elastic

Delayed Elastic

Viscous Flow

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

23 50 60

Str

ain

(in

/in

)

Temperature (C)

Total

Instantaneous Elastic

Delayed Elastic

Viscous Flow

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

23 50 60

Str

ain

(in

/in

)

Temperature (C)

Total

Instantaneous Elastic

Delayed Elastic

Viscous Flow

Figure 4.8a Creep strain components vs. temperature

at activity 0

Figure 4.8b Creep strain components vs. temperature

at activity 0.01

Figure 4.8c Creep strain components vs. temperature

at activity 0.10

Figure 4.8d Creep strain components vs. temperature

at activity 0.35

Figure 4.8e Creep strain components vs. temperature

at activity 0.65

Figure 4.8f Creep strain components vs. temperature

at activity ~1

38

The effect of temperature on creep response can be broken down by looking at the

plots of the different creep components against temperature for each different activity.

Examining the entire set of plots in Figures 4.8a to 4.8f, it can be seen that in general creep

components increase significantly with temperature. The exception to these trends is

activity .10, in which creep components exhibit a maximum at 50 oC. There is very little

difference in creep components for dry Nafion® between 23 and 50 o

C , but a large increase

from 50 o

C to 60 o

C. For all other activities besides 0.10, the different components increased

fairly smoothly with temperature. Not including activity 0 and activity .10, as activity

increases, the difference in creep components between 23 and 50 o

C becomes less

significant and the difference between 50 and 60 o

C becomes more significant. This can be

seen in the plots in the decrease in slope between 23 and 50 o

C and the increase in slope

between 50 and 60 o

C for activities greater than 0.

4.3.3 Discussion

Both temperature and solvent activity strongly affect the viscoelastic response of

Nafion. In general, creep increased significantly with increasing temperature. This is because

bonding strength between polymer chains decreases with increasing temperature, and

therefore the sample creeps more. However, the combined effects of temperature and

solvent activity are complicated, as will be discussed below.

Methanol activity strongly affects the viscoelastic response of Nafion at all

temperatures. Explanations will be given for the creep behavior of Nafion based on changes

in its microstructure with creep conditions. The introduction of methanol results in changes

in the microstructure of Nafion through the acid clusters, teflonic matrix, and bonding

between sulfonate groups. Methanol is protic and possesses a hydrophobic tail. The (polar)

39

protic aspect allows the methanol to undergo hydrogen bonding with sulfonic acid sites,

which are also polar. The non-polar hydrophobic tail can sometimes interact with the non-

polar teflonic matrix. Methanol also disrupts the bonding between sulfonate groups that are

present in dry Nafion due to the formation of stronger hydrogen bonds between methanol

molecules and the sulfonate groups.

Generally, tensile creep strain was found to increase with increase in methanol

activity for all three temperatures. The exception to this trend was runs for methanol

activity 0.10 at elevated temperatures, which showed a very small creep response. This

result was found to be repeatable at both 50oC and 60

oC. As temperature increases, creep

at activity .10 seems to decrease relative to the other activities at that temperature.

Dry Nafion at 23 o

C, exhibited an almost negligible amount of creep. The high

stiffness and resistance to creep of dry Nafion at room temperature is attributed mostly to

cross-linking between the sulfonic acid groups (Majsztrik, 2007). The reason dry Nafion

begins to creep more at 50 and 60 o

C is that at higher temperatures these cross-links are

overcome by thermal kinetic energy. The fact that viscous loss and delayed elastic strain

increase significantly at 60 o

C provides further evidence to support this. As the dry Nafion is

exposed to methanol, the polymer is plasticized, and creep increases with activity.

The general increase in creep strain with methanol activity may be explained in

terms of changes in free volume. Free volume is associated with the space between

molecules in a material. Exposing Nafion to methanol increases free volume due to

methanol’s interaction with both the sulfonic acid clusters and the teflonic matrix. This can

be seen as having the opposite effect of elevated drying temperature, which decreases free

volume. As will be discussed in Section 4.3, elevated drying temperature has an “annealing”

40

effect, which results in decreased free volume because of denser packing of Nafion’s® side

and main chains (Mauritz, 2004). The addition of methanol has the opposite effect, to

increase free volume and thus increase the mobility of side chains. If side chains are more

mobile, the sample will creep more. This explains the general increase in creep strain in

Nafion with activity. This hypothesis for Nafion creep in the presence of methanol is similar

to that of Majsztrik for creep in water at room temperature (Majsztrik, 2007).

Creep increased significantly at activities 0.65 and ~1. This is may be due to

interactions between methanol and the hydrophobic backbone of Nafion. Like water,

methanol will interact mostly with the ionic acid clusters due to its polar –OH group. But

unlike water, under a highly swollen state methanol is also capable of interacting with the

hydrophobic matrix due to its hydrophobic –CH3 group. This was shown in an IR

spectroscopy study of Nafion which found hydrogen bonding interactions between

methanol and –CF2, part of the hydrophobic backbone (Tsai and Hwang, 2007). The study

specified that this interaction occurs only under a highly swollen state, and thus at high

methanol activities (Tsai and Hwang, 2007). At higher activities such as 0.65 and ~1, the

interactions between Nafion and methanol increase free volume through both the acid

clusters and the teflonic backbone. This results in a larger increase in creep strain at these

high activities.

41

Perhaps the most interesting result found was the unusually small creep that

occurred around activity .10 at 50 and 60°C, indicating that there may be some structural

changes occurring in Nafion® around these conditions. Before we discuss the explanation

behind the results found, it is helpful to consider a similar study conducted on tensile creep

of Nafion in the presence of water, given in Figure 4.9 (Majsztrik, 2007). As with methanol,

at 23°C, it was found that water plasticizes Nafion and creep increases with water activity.

Figure 4.9 $afion creep response for different water activities and temperatures (Majsztrik, 2007)

42

Interestingly, creep at 90°C shows the opposite trend: creep decreases with hydration at

90°C. At the intermediate temperatures 50 and 70°C, creep strain exhibits a local maximum

at intermediate water activities.

A similar result is seen with methanol at intermediate temperature; the position of

the local maximum with solvent activity has simply shifted. At 50 and 60 °C, a very

pronounced local maximum in creep strain occurs at methanol activity .01. Thus, the

maximum occurs at a much lower activity than that for water. This shift is due to the fact

that for any given activity, methanol has a greater effect on the morphology of the

membrane because more methanol enters the membrane than water. This is supported by

the fact that methanol produces more swelling in Nafion than water (Benziger, 2008).

The results in both methanol and water can be explained by solute induced changes

of the microphase separation in Nafion. As temperature and solute activity change, the

hydrophilic microphase restructures itself, as discussed in Section 2.2. The possible

equilibrium phases for Nafion are a disordered phase, a BCC cluster phase, a hexagonal

cylindrical phase, and possibly a lamellar phase. These phases can be seen in Figure 4.10, a

phase diagram developed by Matsen and Bates (2004), and adapted by Benziger et. al

(2008). Transitions corresponding to temperature are vertical lines, while transitions

corresponding to changes in water activity at constant temperature are lines that move up

and to the right. It is suspected that changes in methanol activity correspond to lines that

move slightly down and to the right, as indicated by the red line Figure 4.10.

Absorption of a solute such as water or methanol will alter the volume fractions of

the two phases and the interaction parameter between them. Water makes the phases

separate more, while methanol increases the strength of the interaction between the two

phases, or at least moderates the interaction between the two phases. This is due to the

fact that methanol possesses both a hydrophilic

so that it can interact with both phases. This is why the lines corresponding to increasi

methanol and increasing water activity move in different directions.

For dry Nafion or at

phase, and as temperature increases, we follow a vertical line downwards. As temperature

increases, the sulfonic acid groups will break up and randomly distribute themselves in the

Figure 4.10 Phase diagram for microphase separation in block copolymers (Benziger, 2008)

on of a solute such as water or methanol will alter the volume fractions of

the two phases and the interaction parameter between them. Water makes the phases

separate more, while methanol increases the strength of the interaction between the two

at least moderates the interaction between the two phases. This is due to the

ol possesses both a hydrophilic –OH group and a hydrophobic

so that it can interact with both phases. This is why the lines corresponding to increasi

methanol and increasing water activity move in different directions.

For dry Nafion or at very low methanol activity (such as .01), we start in the cluster

phase, and as temperature increases, we follow a vertical line downwards. As temperature

the sulfonic acid groups will break up and randomly distribute themselves in the

Figure 4.10 Phase diagram for microphase separation in block copolymers (Benziger, 2008)

increasing methanol content

43

on of a solute such as water or methanol will alter the volume fractions of

the two phases and the interaction parameter between them. Water makes the phases

separate more, while methanol increases the strength of the interaction between the two

at least moderates the interaction between the two phases. This is due to the

OH group and a hydrophobic –CH3 group,

so that it can interact with both phases. This is why the lines corresponding to increasing

very low methanol activity (such as .01), we start in the cluster

phase, and as temperature increases, we follow a vertical line downwards. As temperature

the sulfonic acid groups will break up and randomly distribute themselves in the

Figure 4.10 Phase diagram for microphase separation in block copolymers (Benziger, 2008)

increasing methanol content

44

continuous TFE phase (Benziger, 2008). This state is referred to as the disordered phase. The

sulfonic acid groups disrupts the crystalline structure which causes an increase in creep rate,

since creep is dominated by the TFE phase. This is the reason that creep for dry Nafion or

very low methanol activities increases with temperature.

Around methanol activity .10, microstructure is probably in the spherical clusters

region. This system remains phase separated. Since the sulfonic acid groups are phase

separated from the TFE in this phase, the TFE’s crystalline structure is preserved, and the

sample actually creeps less than dry Nafion® at the same temperature. As activity is

increased further, the microphase evolves into the cylindrical phase, and at very high

activities into the lamellar phase. In these regions, phase separation is no longer so distinct,

especially in the lamellar phase, causing creep to increase again. Thus, creep continues to

increase with activity up to an activity of around one.

45

4.5 Thermal History Effects

4.5.1 Results

Tensile creep runs were carried out to determine the effects of Nafion’s® thermal

history on its mechanical properties. This was done by varying the time and the

temperature of the drying step. The standard drying procedure for the creep runs in this

thesis was to dry for 3 hours at 50 o

C . For comparison, Nafion® was also dried at 50 o

C for 12

hours, and at 100 o

C for both 3 and 12 hours. Following drying, the samples were cooled to

the test temperature of 23 oC and underwent the standard creep testing for activities 0,

0.10, and 0.65. Results for creep and creep recovery are shown in Figures 4.11 through 4.13.

For the dry runs, drying at 100 o

C resulted in significantly higher creep strain than

drying at 50 o

C. It may be difficult to see in the figures because of the low creep strains

involved, but total creep for dry Nafion after drying for 3 hours at 100 o

C was a factor of five

times larger than that for drying at 50 o

C. This factor was even higher for drying for 12 hours.

For the runs at methanol activity 0.10 and 0.65, the opposite result was seen. Drying at the

higher temperature resulted in a lower creep strain than in samples dried at the lower

temperature. At methanol activity 0.65 , total creep strain for the sample dried at 50 oC was

4 times greater than that for the sample dried at 100 o

C. Results can also be seen in Figure

4.11, which presents an analysis of the differences in creep components for all drying

conditions. These results are consistent with the 2007 findings of Majsztrik for Nafion®

creep in the presence of water.

It was found that drying for longer periods (12 hours versus 3 hours) caused creep to

increase. The largest effect was for activity 0.65, where total creep increased about

threefold when dried for 12 hours for drying at both 50 and 100 o

C. This can be seen in the

46

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 0.1 0.65

Str

ain

(m

m/

mm

)

Methanol Activity

Total Strain12 hrs @ 50°C

12 hrs @ 100°C

3 hrs @ 50°C

3 hrs @ 100°C

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 0.1 0.65

Str

ain

(m

m/

mm

)

Methanol Activity

Instantaneous Elastic Strain

12 hrs @ 50°C

12 hrs @ 100°C

3 hrs @ 50°C

3 hrs @ 100°C

0

0.05

0.1

0.15

0.2

0 0.1 0.65

Str

ain

(m

m/

mm

)

Methanol Activity

Delayed Elastic Strain

12 hrs @ 50°C

12 hrs @ 100°C

3 hrs @ 50°C

3 hrs @ 100°C

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0 0.1 0.65

Str

ain

(m

m/

mm

)

Methanol Activity

Viscous Loss

12 hrs @ 50°C

12 hrs @ 100°C

3 hrs @ 50°C

3 hrs @ 100°C

creep component analysis for all the different drying conditions found in Figure 4.11. Drying

for 12 hours (blue and red bars) consistently produced more creep for all components than

drying for 3 hours (purple and green bars). For the 12 hours runs, it appears that the larger

total strain after drying at 50 o

C is mostly due to an increase in delayed elastic strain and

viscous loss.

Figure 4.11 Creep strain components at 23°C as a function of methanol activity for different drying

conditions, associated with runs shown in Figures 4.12 and 4.13

47

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0 2000 4000 6000 8000 10000 12000

Activity 0 (50°C)

Activity 0 (100°C)

Activity .10 (50°C)

Activity .10 (100°C)

Activity .65 (50°C)

Activity .65 (100°C)

Figure 4.12a $afion creep response at 23 °C for different activities and drying temperatures (dried

for 3 hours)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 500 1000 1500 2000 2500 3000 3500 4000

Activity 0 (50°C)

Activity 0 (100°C)

Activity .10 (50°C)

Activity .10 (100°C)

Activity .65 (50°C)

Activity .65 (100°C)

Figure 4.12b $afion creep recovery at 23 °C for different activities and drying temperatures

(dried for 3 hours)

48

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 2000 4000 6000 8000 10000 12000

Activity 0 (50°C)

Activity 0 (100°C)

Activity .10 (50°C)

Activity .10 (100°C)

Activity .65 (50°C)

Activity .65 (100°C)

0

0.1

0.2

0.3

0.4

0.5

0.6

0 500 1000 1500 2000 2500 3000 3500 4000

Activity 0 (50°C)

Activity 0 (100°C)

Activity .10 (50°C)

Activity .10 (100°C)

Activity .65 (50°C)

Activity .65 (100°C)

Figure 4.13a Nafion creep response at 23 °C for different activities and drying temperatures (dried for 12 hours)

Figure 4.13b $afion creep recovery at 23 °C for different activities and drying temperatures

(dried for 12 hours)

49

4.5.2 Discussion

The results at activity 0 can be explained in terms of bonding within Nafion. In dry

Nafion, ionic groups interact through the electrostatic attraction of S-O-H-O-S groups,

resulting in polymer cross-linking. It is suspected that cross-links between side chains give

dry Nafion® its high resistance to creep at 23 o

C (Majsztrik, 2007). Majsztrik suggests that

between 50 and 80 o

C the strength of this bond is thermally deactivated. Therefore drying at

100 o

C will increase creep at 0 methanol activity because it significantly weakens the cross-

links which normally give dry Nafion® its resistance to creep strain

For activities 0.10 and 0.65, a different effect is seen, due to the introduction of

methanol. The results can be explained in terms of cluster sizes in Nafion®, where the term

cluster is used to describe the hydrophilic domains within the phase-separated polymer. It

has been proposed that high temperature drying results in smaller clusters (Hinatsu, 1994).

Further explanation is offered by Mauritz and Moore: that the decreased volume of clusters

is due to denser packing of side and main chains and microstructure (Mauritz, 2004). In

their 2007 study on the effects of thermal annealing on Nafion®, Hensley et. al. found that

larger clusters will result in higher equilibrium water uptake in a Nafion® film

(Hensley,2007). It is reasonable to infer that larger clusters will result in larger methanol

uptake as well. This thesis has shown that at 23 o

C, creep strain increases with methanol

activity, and thus with methanol uptake. As discussed above, elevated drying temperature

results in smaller acid clusters, and therefore smaller methanol uptake. If methanol uptake

is decreased, as in those samples dried at 100 o

C, it is reasonable to assume that creep strain

will be decreased as well.

50

The results for drying time may be explained by microphase restructuring in Nafion.

This process is a mixture of both thermodynamics and kinetics, thus, it is a time dependent

process. If the membrane is dried for a longer period of time, it becomes more disordered.

As the membrane is cooled back down, it begins to revert back towards its original state,

however, since the membrane was dried for such a long period of time, it also takes a long

time to revert back. Therefore the sample is not as stiff, and more creep occurs. This effect

is greater for higher temperatures.

4.5 Solvent Effects

The tensile creep response of Nafion® was investigated using a 50/50 mixture of

methanol and water by mol fraction. This was done mostly to examine the synergistic

effects of methanol and water on creep response. However, it also provides a method to

probe deeper into the interactions occurring in Nafion® which give it its unique mechanical

properties. Results for the methanol-water mixture are also compared to results with pure

methanol, as well as those with pure water, courtesy of Majsztrik (2007). Volumes were

calculated to achieve the desired activity based on the assumption of Raoult’s Law in vapor-

liquid equilibrium.

Creep strain curves for the methanol/water mixture at 23 and 60 o

C are given in

Figure 4.15 and 4.16, respectively. For comparison, results from Majsztrik’s 2007 study of

Nafion® creep at 23°C and 50°C when exposed to water vapor are presented in Figures 4.17-

4.18. Unfortunately, there are no results for water at 60°C , although general comparisons

can be made at these intermediate temperatures. The creep behavior for methanol seen at

23 o

C is as expected, creep increases with increasing activity. Comparing creep at room

temperature for pure methanol, the 50/50 mixture, and pure water, we see that creep is

51

very similar for pure methanol and for the methanol/water mixture (See Figure 4.14). We

notice that a very similar amount of creep occurred at lower activities for pure methanol

and pure water. However at activities .65 and ~1, more creep occurred for methanol than

water.

Interestingly, at 60°C, creep was highest for dry Nafion and lower when methanol

was introduced. However, creep still increased with increasing methanol activity, although

the curves were very close. This inversion is similar to the results seen for pure water at 50

and 70°C by Majsztrik. Total strain for 100% methanol, 50% methanol/50% water, and 100%

water at 23°C is reported in Figure 4.14. Results at 60°C for methanol will not be directly

compared to those for water, as we only have results for water at 50 and 70°C.

0

0.05

0.1

0.15

0.2

0.25

0.01 0.1 0.35 0.65 ~1

To

tal

Str

ain

(m

m/

mm

)

Activity

100% methanol

50% methanol/50% water

100% water

Figure 4.14 Comparison of total creep strain at 23°C for the three solvent types

52

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 2000 4000 6000 8000 10000 12000

activity 0

activity .01

activity .10

activity .35

activity .65

0

0.02

0.04

0.06

0.08

0.1

0.12

0 500 1000 1500 2000 2500 3000 3500 4000

activity 0

activity .01

activity .10

activity .35

activity .65

Creep strain at room temperature is higher for pure methanol and methanol/water

Figure 4.15a $afion creep response at 23 °C for 50/50 methanol/water mixtures (by mol fraction)

Figure 4.15b $afion creep recovery at 23 °C for 50/50 methanol/water mixtures (by mol fraction)

53

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 1000 2000 3000 4000

Str

ain

(m

m/

mm

)

Time(s)

activity 0

Activity .01

activity .10

activity .35

activity .65

0

0.05

0.1

0.15

0.2

0.25

0.3

0 2000 4000 6000 8000 10000 12000

Str

ain

(m

m/

mm

)

Time (s)

activity 0

activity .01

activity .10

activity .35

activity .65

Figure 4.16a $afion creep response at 60°C for 50/50 methanol/water mixtures (by mol fraction)

Figure 4.16b $afion creep recovery at 60°C for 50/50 methanol/water mixtures (by mol fraction)

54

Creep strain at room temperature is higher for pure methanol and methanol/water

mixtures, and lower for pure water. These results can be explained by the differences in

interactions between Nafion® and the different solvents. Interaction between water and

Nafion® is restricted entirely to the polar acid clusters as opposed to the non-polar matrix,

because water is highly polar. When methanol enters the Nafion®, it will also preferentially

interact with the hydrophilic acid domains because of its polar -OH group. However, unlike

water, under a highly swollen state methanol is also capable of interacting with the

hydrophobic matrix due to its hydrophobic –CH3 group. An IR spectroscopy study of Nafion

found hydrogen bonding interactions between methanol the deprotonated sulphonic group

in the hydrophilic domains at low methanol activities and with –CF2, part of the hydrophobic

backbone, at higher activities. (Tsai, 2007). These results are also supported by the fact that

methanol produces more swelling than water (Majsztrik, 2007).

The fact that methanol/water mixtures behaved very similarly to pure methanol,

especially at higher activities, could be due to the preferential uptake of methanol. Studies

found that at low methanol concentrations the uptake of methanol is little affected by the

uptake of water, but at higher methanol concentrations the water starts to become

excluded (Skou, 1997). At these higher activities, the membrane is becoming mostly

saturated with methanol, and therefore behaves as it would if it were exposed to pure

methanol.

The results at 60 o

C provide further support the theory of microphase separation in

Nafion. The result seen here is very similar to that seen for water at 50 and 70 o

C.

Unfortunately, there are no results for Nafion creep in water at 60 o

C for an exact

comparison. However, the inversion at intermediate temperatures for dry Nafion and the

local maximum with solute activity is seen. This may be seen as a shift in the temperature of

the local maximum seen with solute activity, or a shift in the activity of the local maximum

itself.

Figure 4.17. $afion® creep respo

Figure 4.18. $afion® creep response

local maximum with solute activity is seen. This may be seen as a shift in the temperature of

the local maximum seen with solute activity, or a shift in the activity of the local maximum

creep response at 23oC for various water activities. (Courtesy of

. $afion® creep response at 50oC for various water activities. (Courtesy of

55

local maximum with solute activity is seen. This may be seen as a shift in the temperature of

the local maximum seen with solute activity, or a shift in the activity of the local maximum

various water activities. (Courtesy of Majsztrik)

(Courtesy of Majsztrik)

56

5. Conclusion

An understanding of a fuel cell membrane’s mechanical properties is essential for

improved performance and longevity of a PEM fuel cell. Nafion’s® mechanical properties

depend strongly on temperature and solvent activity, and thus, the viscoelastic response of

Nafion® was investigated at controlled temperature and solvent activity. Tensile creep and

membrane swelling were measured using a unique apparatus constructed by Paul Majsztrik

which allows creep measurements to be taken at precisely controlled temperatures and

solvent activities. The effects of thermal history and different solvents on creep were also

investigated.

In general, increased temperature increases creep by decreasing the bonding strength

between polymer strains. However, the combined effects of temperature and solvent

activity are more complicated. It was shown that at room temperature, increased methanol

activity increases creep. Methanol interacts with both sulfonic acid sites and the teflonic

backbone of Nafion, which affects the polymers microstructure and bonding interactions.

The increase in creep with activity is due to the increased free volume with occurs with the

addition of methanol, and thus increased mobility of side chains, which causes more creep

to occur. The significantly higher creep strain that occurred at high methanol activities is

suspected to be due to the interaction between methanol’s non-polar –CH3 group and the

hydrophobic teflonic backbone of Nafion.

At intermediate temperatures 50 and 60 o

C, a local maximum in creep was seen at

methanol activity .01. Creep then decreased significantly at activity .10 and increased

thereafter. These results are actually similar to those found by Majsztrik for Nafion creep in

water, in which a local maximum at intermediate activities was found for these

57

temperatures. In methanol, the position of the local maximum with activity has simply

shifted. These results can be explained by solute induced changes of the microphase

separation in Nafion. As temperature and solute activity change, the hydrophilic microphase

restructures itself. The possible equilibrium phases for Nafion are a disordered phase, a BCC

cluster phase, a hexagonal cylindrical phase, and possibly a lamellar phase. For methanol, at

intermediate temperature and activity .10, we are in the spherical cluster phase. Since the

sulfonic acid groups are phase separated from the TFE in this phase, the TFE’s crystalline

structure is preserved, and the sample actually creeps less than dry Nafion® at the same

temperature. However, as activity is increased further, the microphase evolves into a

cylindrical phase and then a lamellar phase, in which more creep occurs due to less phase

separation. The results from methanol-water mixtures exhibited a similar result at

intermediate temperatures between that of methanol and water, confirming the hypothesis

of microphase separation.

Nafion’s mechanical properties were also shown to be strongly dependent on

thermal history. For the dry Nafion, increased creep strain after drying at higher

temperatures is due to the thermal deactivation of S—H cross-links which normally give dry

Nafion® its high resistance to creep strain. The lower creep strain for activities 0.10 and 0.65

for the higher drying temperature can be explained in terms of the acid cluster sizes of

Nafion. It has been proposed that higher drying temperature results in smaller acid clusters

due to denser packing of side and main chains. Smaller clusters will result in a smaller

methanol uptake by the membrane, which means less creep will occur for a higher drying

temperature. Increasing drying time increased creep. If the membrane is dried for a longer

period of time, it does not have time to revert to its ordered state, and more creep occurs.

58

The final morphology of Nafion is a function of solvent type and activity, temperature,

and thermal history. Tensile creep response has been investigated varying all of these

factors in order to gain a better understanding of Nafion’s morphology under different

conditions, and its effect on the polymer’s mechanical properties. It was found that all of

factors had a significant effect on mechanical properties. Since the mechanical properties of

the polymer electrolyte membrane influence the performance, longevity and durability of

the fuel cell, it is very important to have a good understanding of these properties under the

actual operating conditions of the fuel cell. Further work should investigate how Nafion

could be modified to improve its performance under different environmental conditions.

Additionally, experiments could be repeated with other solvents in order to gain a better

understanding of the theory of microphase separation in Nafion.

59

This paper represents my own work in accordance with University regulations.

Christine Ranney

60

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